Upon cooling and solidification of molten metal core volume decreases. As a result, in the central part of the nucleus can be formed piping, porosity and friability of the metal. The thicker the metal, the greater the adverse effect of shrinkage and the greater the probability of defect formation. The most reliable way to deal with them is to increase the operating pressure, and the transition to the cycles of welding forging.
Usually in the welded joint is located a few points, so that the welding has to be the diversion of current through a previously welded points, shunt point to be welded. Having previously welded points is decreased efficiency of the electrodes on welded pressure point, as part of this pressure is perceived earlier welded points. Therefore, the welding of several closely spaced points of the average strength of the points is lower than when welding a single point. The strongest point in the node is usually the first one.
For spot welding the metal contamination in the weld zone must first be carefully removed by brushing, pickling in acid, opeskostruivaniem etc. Build a spot welding should as closely as possible to ensure a snug fit of parts before welding. The presence of the gap between the components absorbs a significant part of the pressure on the deformation of the electrode parts to solid ground, the actual pressure on the sedimentary point becomes insufficient and the strength of the scatter of points is obtained. The requirements for precision assembly increased with increasing thickness of the sheets.
There are so-called soft and hard modes of spot welding. When the soft modes are moderates current, current density at the working electrode surface is typically less than 100 A / mm 2 . For hard modes of the current density reaches the welding of steel to 120-300 A/mm2.
The soft modes are characterized by a greater length of time of welding, smoother heating, reduced power welding. The advantages of soft modes include reducing the power consumption of the network, reducing network load, lower power and cost of the necessary machinery of contact, reducing the hardening of the fusion zone. Rigid regimes require high power machines, increase the maximum load on the network. The advantages of hard welding conditions include reducing welding time, increase productivity. The pressure of the electrodes is usually taken within 3.8 kg / mm 2 .
Improperly installed or violation of the welding process requirements may lead to a variety of spot welding defects. The most dangerous is the lack of fusion defect, characterized by the absence of molten core point or small by its size. Risk of lack of penetration is increased by the fact that it is not always reliably detected by visual inspection of products at acceptance. May occur as defects such as surface and podplavlenie burn-through metal, deep indentations on the surface of the metal shell and the porosity of the molded core.
Spot welding is mainly connected components of low carbon steel having excellent weldability. Alloy steels are prone to hardening and steel with high carbon content should be welded to the soft modes. When welding on hard regimes the core point and the surrounding area of influence is strongly quenched and exhibit an increased tendency to form cracks. High-strength steel demand to increase the working pressure during welding.
After welding, heat treatment of products is sometimes necessary to remove the internal stresses generated by the welding process, or to improve the structure of the metal, mainly for the destruction of the structure of martensite is especially dangerous. Typically, heat treatment is reduced to a high holiday. Most subsequent heat treatment can be repeated by passing the current directly to the point the car immediately after the welding point.
Good welded austenitic stainless chromium-nickel steels of 18-8. To reduce the decomposition of austenite and the precipitation of carbides are welded on hard regimes, with the least possible time welding. Apply high pressure, requiring electrodes of high-strength alloys. Welding time is reduced to 0.01 seconds per point for the thin material. Can spot welding of aluminum, aluminum and magnesium alloys. Due to the high thermal and electrical conductivity of aluminum for its spot welding requires a large current density at the electrodes, reaching in some cases, the 1000-1500 A / mm 2 . In this case the aluminum particles easily stick to the electrodes and the particles of copper electrodes stick to the aluminum sheets. To reduce the adhesion requires a thorough cleaning of the surfaces of sheets and the working surface of electrodes. Aluminum alloys are usually welded together a little better commercially pure aluminum due to increased electrical resistance.
See also:
Electric Resistance Welding
Pliers for Welding
Spot Welding
The Spot Welding Process
The Spot Welding Process (Part 2)
Special Types of Spot Welding
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